In smaller electrical transmission grids, like micro-grids, the frequency and voltage are subjected to larger variations than in a large grid. Micro-grids often rely on power from a larger main grid to provide balanced voltage. Especially, balancing problems may occur if a micro-grid loses a connection to a main grid.
US2012/0283890 (D1) describes a control apparatus for a micro-grid. D1 discuss controlling a micro-grid when the micro-grid is disconnected from a main grid. If power from the main grid is interrupted, a control system may increase the power production and reduce the power consumption of the micro-grid (see FIG. 1 and §37). When power from the main grid is lost, a local controller enters islanded operation from a grid-connected operation (§43-44).
US2012/0283888 (D2) describes an embodiment of the apparatus of D1, wherein a power generation plan is created by a central controller for an islanded operation in order to lessen the balancing problems when the micro-grid is disconnected from a main grid and changes to islanded operation (see abstract, §43). The power generation plan for islanded operation includes increasing power supply and reducing load by load shedding (§45-46). The power generation plan tries to match power supply and load (§45). According to D2, the execution of the power generation plan by the local controller should be performed in a transition being less than 0.6 seconds.
However, the transition phase is the most critical when moving from grid-connected into islanded operation. Execution of a power generation plan that matches supply and load for islanded operation, as suggested in D2, is not optimal for the transition phase. Regulating power supply and perform load shedding during the transition in the same way as matching during islanded operation may also cause balancing problems due to the interaction of the power supply regulation and load shedding operations performed simultaneously during the transition phase. Moreover there is a need for performing a very fast response when the power supply from the main grid is lost. D1 and D2 do not provide details on providing a fast response and no details of the load shedding and power supply being specific for the transition phase.